The present invention relates to a solid state area imaging apparatus which converts two-dimensional light information into an electric signal.
The typical conventional opto-electro function part of solid state area imaging device is a p-n photodiode or a MOS photodiode. Such opto-electro function part is of a charge accumulation type wherein carriers generated by the incident light are accumulated or stored in a depletion region which is in an electrically floating state, and thereafter the accumulated carriers are read out in sequence by scanning on the two-dimensional imaging device by a picture element selecting means and a signal readout means. The photo-sensitivity of such imaging device depends upon the aperture index of photo-detector.
For this reason, where a higher integration density such as smaller chip size, larger number of picture elements and the like of the imaging device is developed, it is unavoidable that the photo-sensitivity is reduced in proportion to the reduction in the aperture index of the photo-detector.
Furthermore, on the other hand, so far as the photo-electro function part is of a charge accumulation type, counter measures against blooming becomes indispensable because an excessive incident light causes a blooming effect.
In such circumstances, as a device structure enabling an increase in photo-sensitivity and elimination of blooming, solid state area imaging apparatuses have been proposed wherein a photo-conductive film, which is an insulator, is deposited on the surface of a semiconductor substrate. Opto-electro functionless and charge accumulation (or storing) parts of picture elements (p-n diode, etc.), picture element selecting means (scanning circuit, etc.), and a signal readout means [signal transport line, CCD (charge coupled device), etc.] are formed on the semiconductor. (For instance, Japanese Unexamined Patent Publication No. Sho 49-91116, Japanese Unexamined Patent Publication No. Sho 51-10715, Japanese Unexamined Patent Publication No. Sho 51-95720, Japanese Unexamined Patent Publication No. Sho 51-95721, etc.). However, the proposed apparatuses have many problems in performance, because the photo-conductive layer is constituted primarily with amorphous semiconductor and polycrystalline semiconductor, and also the perfectness (purity, precision of stoichiometry ratio, and crystalline perfectness) of these materials is inferior to single crystalline silicon.
In particular, in the case with amorphous semiconductor, there exists a problem of being subject to a change in characteristics because crystallization is likely to occur due to its thermal instability. And in the amorphous semiconductor, also exists a problem of extremely slow speed of response because the trap density in the bandgap is far larger in comparison to that of single crystalline silicon, resulting in a reduced mobility of carriers.
Furthermore, problems in the case with polycrystalline semiconductor are that the precision of grain size (normally about several .mu.m) has an effect on reproducibility of characteristics at manufacture. That is, the grain size becomes about the same as the mean free path of electrons, and control of characteristics becomes difficult due to a change in band structure.
On the other hand, there are also proposals for carrying out countermeasures against blooming by increasing sensitivity by employing an opto-electro function part having an amplification function in the device rather than by increasing the sensitivity by improving the aperture index. In this proposed way, the opto-electro function is performed by means of depletion mode operation of majority carriers in the picture element part (For instance, Japanese Unexamined Patent Publication No. Sho 55-124259). The opto-electro function part in this case is a phototransistor of SIT (Static Induction Transistor: see IEEE Trans. Vol. ED-22, Nov. 4 (1975) pp. 185-197) type and has an intrinsic base or intrinsic gate in a potentially floating state, and therefore a switching function for resetting unrequired charges accumulated at that part (this corresponds to the minority carriers in the picture element part) is required. Accordingly, it is extremely difficult to realize the necessary number of picture elements (for instance, 200,000 picture elements are arranged in matrix on an imaging area of only about 2/3 inch diameter, horizontal 400.times.vertical 500=200,000) as of the conventional solid state area imaging apparatus wherein design has been carried out by the rule of 2.5 to 3 .mu.m.